Control Notes

I was recently asked for my input on control loop performance by means of a few questions.  Here is a somewhat modified version of the questions and answers.

Q: What is the percentage of control loops that operate properly in the average process plant?

A: Two papers dating back to 1993, authored individually by Ender [1] and Bialkowski [2], claimed that roughly 30% of loops were left in manual, 30% actually increased variability, and that only 20% of loops performed well. In 2001 Desborough and Miller [3, 4] confirmed that control loop performance was on average still the same.  In 2008, VanDoren [5] also reported a very similar bleak state of loop performance.
Our experience from loop performance audits supports these findings, although the distribution between loops in manual, those with poor performance, and those with good performance, can vary significantly between different sites and process types.

Q: When does a loop operate properly?

A: Proper operation can be subjective, and it is easier to define improper operation. Loops that increase variability when running in auto compared to manual control would not be considered as operating properly. Nor would controllers reacting mainly to noise and high-frequency disturbances, or those causing oscillations in the loop.  Controllers with outputs running into limits, and controllers tuned too sluggishly are also not operating properly. So, for a loop to be “operating properly,” it needs to reduce variability, and do so in a repeatable fashion, consistent with its function in the larger process.

Q: What can cause control loops to operate poorly?

A: Improper operation can be caused by incorrect controller tuning settings, incorrect process variable (PV) filter settings, faulty or incorrectly positioned instrumentation, or mechanically defective, non-linear, oversized, or undersized final control elements. It can also be caused by not making use of an appropriate control strategy (like feedforward or gain scheduling), or improper design of such a control strategy (like dividing one flow by another for doing ratio control).

Q: What are the consequences poor loop performance?

A: A poorly performing control loop can decrease product quality, limit maximum production rates, extend process start-up and transition times, increase the likelihood of unplanned process shut-downs, increase maintenance costs, consume more energy, and make the process difficult to operate.

Q How should control loop problems be resolved?

A: The first step is to find the problem loops.  Some problematic control loops are obvious due to their impacts on operations, while others might be less obvious or remain hidden to operators and process engineers.  A comprehensive list of control problems can be obtained most effectively by using software to assess the performance of all control loops.  The second step is to distill the performance survey down to a list of “bad actors” by looking both at the performance and the relative importance of each control loop. Bad actors would be loops performing poorly and also significantly impact the process. Loop assessment software can do this automatically if they are configured properly.  The third step is to diagnose the root cause of the problems.  Loop assessment software is helpful in doing this, but engineering and process knowledge is also required.  The final step is fixing the problem according to the diagnosis, for example by tuning a controller, fixing a control valve, or implementing the appropriate control strategy.

Q: Why are some control loop problems so persistent?

A: Control problems persist mostly because the root causes of the problems are not being addressed. For example: An engineer may spend hours tuning and re-tuning a control loop, but his efforts are futile if the problem actually is a sticky control valve requiring maintenance, or a nonlinear process requiring gain scheduling.

Q: What is the most important factor when optimizing control loops?

A: The single biggest factor in loop optimization is the skill level of the person doing the optimization work.  Software has come a long way to simplify loop performance analysis and tuning, but the software is just a tool, and the validation of its diagnoses and execution of corrective actions need to be done by an adequately skilled human. Skills can either be hired, or learned.  In both cases OptiControls can help with our services and training.

Stay tuned!
Jacques Smuts – Author of the book Process Control for Practitioners

References:
[1] D.B. Ender, Process Control Performance: Not as Good as You Think, Control Engineering 40 (10), 1993, 173 – 186.
[2] W.L. Bialkowski, Dreams Versus Reality: A View From Both Sides of the Gap, Pulp & Paper Canada, 1993, 94 (11), 19 – 27.
[3] L. Desborough and R. Miller, Increasing Customer Value of Industrial Control Performance Monitoring: Honeywell’s Experience, Proc. 6th Int. Conf. on Chemical Process Control (CPC VI), Arizona, USA, 2001, 172–192.
[4] L. Desborough, P. Nordh, R. Miller, Control System – Process out of Control, Industrial Computing, August 2001, 52 – 55.
[5] V. VanDoren, Advances in Control Loop Optimization, Control Engineering, March 2008, 48 – 52.